text: It was concluded that bacterial deposition mainly occurred in a secondary energy minimum  by using DLVO theory. DLVO calculation predicted an energy barrier of 140kT at 31.6 mM ionic strength to over 2000kT at 1mM ionic strength. This data was not in agreement with the experimental data, which showed increasing deposition with increasing ionic strength. Therefore, the deposit could occur at secondary minimum having the energy from 0.09kT to 8.1kT at 1mM and 31.6 mM ionic strength, respectively. The conclusion was further proven by the partial release of deposited bacteria when the ionic strength decreased. Because the amount of released bacteria was less than 100%, it was suggested that bacteria could deposit at the primary minimum due to the heterogeneity of the surface collector or bacterial surface. This fact was not covered in classical DLVO theory.<br />
  The presence of divalent electrolytes (Ca) can neutralize the charge surface of bacteria by the binding between Ca and the functional group on the oocyst surface. This resulted in an observable bacterial deposition despite the very high electrostatic repulsive energy from the DLVO prediction. 
  The motility of bacteria also has a significant effect on the bacterial adhesion. Nonmotile and motile bacteria showed different behavior in deposition experiments. At the same ionic strength, motile bacteria showed greater adhesion to the surface than nonmotile bacteria and motile bacteria can attach to the surface of the collector at high repulsive electrostatic force. It was suggested that the swimming energy of the cells could overcome the repulsive energy or they can adhere to regions of heterogeneity on the surface. The swimming capacity increase with the ionic strength and 100mM is the optimal concentration for the rotation of flagella. 
  Despite the electrostatic repulsion energy from DLVO calculation between the bacteria and surface collector, the deposition could occur due to other interactions such as the steric impact of the presence of flagella on the cell environment and the strong hydrophobicity of the cell.